The invention relates to an improved means for determining the ultimate compressive properties, the compression required to cause fiber failure, of carbon, graphite, and other conducting fibers which may be used in advanced composites.
Previous methods and apparatus for determining the ultimate compressive properties of conducting fibers used in advanced composites all suffer from one or more deficiencies. The elastica loop test, as described by W. R. Jones and J. W. Johnson in Carbon Volume 9, pg. 645 (1971), is simply a bending test for a single fiber that provides a value of strain to failure. The failure from such a test may be either compressive or tensile, and material nonlinearity can contribute to errors in the measured failure strain. Furthermore, tedious measurements of the loop geometry are required to determine the strain in the fiber at the failure point. A more recently developed tensile recoil test, as described by S. R. Allen in the Journal of Material Science, Volume 22 (3) starting at page 853 (1987), is based on compressive stresses that are generated during recoil of a fiber after tensile fracture. Although this test can potentially yield values of fiber compressive strength, it is only applicable to fibers exhibiting tensile strength greater than compressive strength. Furthermore, the rate of deformation in the recoil test is both extremely high and uncontrollable and fiber bending during recoil may result in premature failure and anomalously low values of compressive strength. These problems encountered with this test are further described by C. S. Wang, S. J. Bai, and B. P. Rice in the Proceedings of the ACS Division of Polymeric Materials: Science and Engineering, Volume 61, beginning at page 550 (1989). Attempts have been made to calculate fiber compressive strengths using strengths of composites containing a multiplicity of fibers in a matrix binder. Unfortunately, it is extremely difficult to determine the mode of failure in a composite and to determine the effects of matrix properties, fiber-matrix adhesion and imperfections on the measured composite strength. Additionally, composite strengths measured using different test methods for one material can vary widely. Consequently, very little information can be gleaned from composite tests alone.
H. M. Hawthorne and E. Teghtsoonian in The Journal of Material Science, Volume 10(1) beginning at page 41 (1975) describe a fiber compression test which involves compression of a fiber embedded in a transparent matrix. In this test, the strains in the matrix and the fiber are identical and can be easily measured on the surface of the matrix. Fiber failure must be detected by visual observation. The advantages of this test include uniform compression of the fiber, controllable deformation rates and results that are independent of material nonlinearity. Major drawbacks of this test are the unknown residual strains in the fiber resulting from shrinkage of the matrix during the embedding process and the tedious effort required to visualize fiber failure which can occur anywhere along the length of the embedded fiber.